The Keys skipper made some stand out performances to earn himself the honour and head coach Greg Woods believes Nash has been an integral part of the team's recent successes.
"Rob has been fantastic this season," he said. "He is a great tackler and ball carrier, leads by example and really gets up for the big games which is all you can ask for as a coach. He's all-action in attack and defence and has really come into his own this season as captain. What's more, he can finally cut his hair now that we've won a trophy!"
Woods has long been a big believer of the Principality Premiership, nurturing the likes of Jevon Groves, Steffan Jones and Lloyd Burns on to regional rugby.
"The Principality Premiership is getting stronger every year, you can't take anyone for granted now and it's certainly a great breeding ground for talent. Rob is one of the outstanding performers in the league this season and deserves every recognition for that."
"To finish in the top six in the Premiership, reach the British & Irish Cup final and win the SWALEC Cup means it's been an incredible season, one we will all remember for a long time."
Nash, who played junior rugby for Panteg before moving on to Pontypool United and Pontypool said: "It's an honour to receive this award - I've looked on the WRU website and a lot of established players are on the roll of honour for the Principality Premiership Player of the Month so I'm really pleased.
"All three coaches at Cross Keys have been great to me but I have to thank Jonathan Westwood, the Cross Keys Director of Rugby, as he really believed in me when I first arrived at the club. He clearly saw something in me and ensured I got a chance to shine. Although I captained the side last year, I feel I've grown into the role this season.
"I think you make your own luck to a certain extent and it's fantastic that our efforts this season have been rewarded with some silverware.
"I would jump at the chance of being a professional rugby player and certainly wouldn't turn down an offer if it came in but at 25 I feel the opportunity has probably now passed me by. I'm very happy at Cross Keys, we are all best friends here and to cap off the season with the cup win, a place in the British & Irish Cup final and to finish in the top six in the Premiership is fantastic."
Robert Nash Factfile
Born: 23.8.86, Panteg
Club: Cross Keys
Position: Openside flanker
Most admired player: 'Sam Warburton is a class No 7'
Education: West Mon Comprehensive, Pontypool; Cwmcarn Sixth Form